Electrolytic protective surface treatment of magnesium base alloys



Jan. 18, 1949. E, COATES 2,459,365

ELECTROLYTIC PROTECTIVE SURFACE TREATMENT OF MAGNESIUM BASE ALLOYS Filed Feb. 21, 1944 2 Sheets-Sheet 1 Fig.1.

ZAL.

CORROSION RATE //v EMP/P/CAL LIN/7.6 N N c Ch z o Q0 0 O 10 2 O 3 O ANODI: CURRENT DEMS/TY A MP5 PER 5 QUA RE FOO 7 v Jwewfaf: g eaff/gg/i'dzfla/z/ aafes Jan. 18', 1949. G. E. COATES 2,459,365

ELECTROLYTIC PROTECTIVE SURFACE TREATMENT OF MAGNESIUM BASE ALLOYS Filed Feb. 21, 1944 2 Sheets-Sheet 2 CORROSION RA TE IN E'lW-"II-F/(JAl UNI T8 6 TIME OF TRE'A TNEN T IN M/NU TES.

o ANODE CURRENT DENSITY /N ANPERES PER SQUARE FOOT.

4- 6 8 TIME IN MINUTES.

Wye/7227' Patented Jan. 18, 1949 ELECTROLYTIC PROTECTIVE SURFACE TREATMENT OF MAGNESIUM BASE ALLOYS Geoffrey Edward Coates, Swansea, Wales, assignor, by mesne assignments, to The Permanente Metals Corporation, a corporation of Delaware Application February 21, 1944, Serial No. 523,296 In Great Britain April 3, 1942 1 Claim. 1

This application, which is .a continuation in part of pending United States application Serial No, 469,444 of Geoifrey Edward Coates, filed December 18, 1942, for Treatment of magnesium base alloys, is for an invention which relates to the electrolytic protective surface treatment of magnesium base alloys, especially but not exclusively those alloys prepared according to Specifications of the British Air Ministry Department of Technical Development hereinafter referred to as D. T. D. specifications.

For purposes of clarity, some material :already set out in the aforesaid specification No. 469,444 herein referred to as the main case is repeated herein. As in the main case, this application refers to protection by methods utilising dichromate solutions.

The main functions of such a protective treatment are two-fold:

(a) To protect articles whether in the form of castings, sheet, extrusions, forged pieces or other forms during service, or semi-finished products during transit and storage before machining or fabrication, including protection Without a paint or like coating.

(b) To serve as a base for paints, or the like, providing with the paint a more permanent protection in service.

It has long been known that dichromates inhibit the corrosion of magnesium. Thus Lohnstein (Zeit. Elektrochemie, 13, 1907, p. 613) showed that the corrosion of a magnesium surface may be inhibited by the addition of a dichromate to the corroding medium, and Dunstan .and Hill (J. C. S., 96, 1911, p. 1835) showed that a 1 solution of potassium chromate or dichromate would render magnesium passive so that the metal was relatively unattacked by water,

The protection afforded by dichromate solutions is due to the formation of a protective film of reaction products on the surface of the metal, but simple dichromate solutions are of little practical value ecause both the rate and total extent of film formation are severely limited.

Amongst what may be described as the dichromate type of treatments for the corrosion protection of magnesium base alloys, recourse has been made to a variety of methods in endeavours to overcome the above difficulties but no hitherto published process combines all the desired attributes of a commercial protective treatment which may be summarized as the combinationof the following features or requirements:

(a) The treatment should cause the minimum dimensional change so that machined articles may be treated without disturbance to dimensional tolerances, .for instance, less than can be reasonably measured with a thousandths of an inch micrometer.

(-b) The raw materials for compounding the solution should be cheap and of innoxious character, e. 5:. strongly acid or alkaline solutions should be avoided and the rate of consumption of the ingredients in operation should be low.

(c) Time of treatment should be as short as possible, i. e. preferably not more than a few minutes and less than half an hour.

(at) Theprocess shouldbe capable of operation at room temperature,

(e) The process s'hould'be characterised by ease and economy in operation and maintenance and the solutions should be stable.

In my application No. 4653,4441 showed how these desiderata can be more successfully attained than in any hitherto known process and that for the first time the full combination can be secured namely by careful control of solution composition both as regards the particular ingredients and their concentration. One of the special attributes of the solutions described'in that application is the short treatmenttime required, and it was revealed that the same degree of protection can be secured in an even shorter time by heating the solutions. In that application I dealt with both chemical processes and. with electrolytic processes. I have found that for the same degree of protection the treatment time can be reduced by the electrolytic method in comparison with'the chemical method, namely, by submitting the Mg allo workpieces to an electrolytic treatment in solutions of identical constituents but of slightly different sulphate concentration range. Alternatively, an enhanced degree of protection can be secured by applying the electrolytic treatment for the same treatment time.

Several electrolytic treatments have been proposed for the protection of magnesium alloys but none satisfy the requirements which .I have found essential to secure the advantages described above. These I find can only be secured by utilisation of the following new combination, namely, a dichromate plus a sulphate plus acetic acid or some other buffering agent. The buffering agent must, however, be properly chosen, for instance,

it must be one that does not form precipitates with the other electrolyte ingredients.

The object of the present invention as in part follows from what has been said above, is to provide an improved electrolytic process for the throughthe pores. Lcorresponding anions can diffuse into the pores to permit further dissolution of Mg and conprotection of Mg base alloys in dichromate solutions, especially from the aspect of the following features in combination, namely, short-time treatment, capacity for treatment at substantially room temperatures and under relatively low voltage conditions to minimise the need for safety precautions, avoidance of strongly acid solutions which in addition to being unpleasant to use are likely to cause undue dimensional loss, and employment of economical stable solutions.

These objects are achieved by electrolytic treatment in solutions, the particular ingredients and concentrations of which, although some of the features of themselves or in other combnations as indicated above are to be found in the prior art, constitute an essential and entirely new combination in respect of the features: dichromate concentration, sulphate concentration, hydrogen ion concentration, and a buffer agent,

the cations of all salts used being chosen to meet particular requirements.

The new combination consistsof the following essential features (a) to (d)'namely:

(a) Dz'chromate ions in concentration not less than 0.05 molar and preferably between 0.2 molar and 0.5 molar, which region is the optimum when the other factors are at their optima.

(b Sulphate ions in concentration not less than 0.5 molar and preferably between 0.1 molar and 1.5 molar which region is the optimum when 1 the other factors are at their optima. The presenoe of sulphate ions is essential for reasons tentatively explainedas follows:

When a piece of Mg is immersed in a dichrowhich though not clearly understood, may be mate solution, the Mg reacts with the dichromate 1 and the reaction products form a film on the surface which inhibits further attack, but which is too thin to be of practical value when the specimen is withdrawn. The'film is believed to be highly porous and film formation would proceed further, if Mg cations could diffuse outwards They can onl do this if tinued. film formation byreaction with dichromate. pea'rsthere must be a balance between the inward difiusion of the anions and the rate at which the protective reaction products can be formed. The rate at which the anions can diffuse into the pores depends on their size and charge. Sulphate anions are the only suitable ones in these two respects. Chloride anions are too small and therefore diffuse in too quickly, causing pitting as observed in practice. The

same is true to a lesser extent of bromide anions.

Iodide anions might be suitable but are ruled out because they are oxidised by the dichromate.

Nitrate anions are about the same size as sulphate anions but have only a single charge rendering them relatively ineffective, which I have confirmed experimentally} Nitrate anions have a single charge and would probably be of the right size but are oxidised by the dichrornate. All other anions are either too large or form insoluble reaction products with Mg which clog the pores and stop further film formation.

I have found that not only is it essential to 'have sulphate anions present, but also that to be effective, their concentrationmustbe not less than 0.05 molar, better results being obtained between about 0.1 and 1.5 molar. Above this range, there is danger of sulphates crystallising For satisfactory film formation, it apsional tolerances.

out and interfering with the successful operation of the process.

(c) Hydrogen ion concentration.lt has long been known that if the acidity of dichromate solutions is increased, therate of protective film formation is also increased. Unfortunately the shows the relationship between the hydrogen ion concentration and percentage of Al in alloys treated electrolytically in solutions of lowest concentration in the optimum range given in Table IV below. All values, however, fall withinthe extreme pH range4-6.

The pI-I values quoted refer to the glass electrode. Other methods of pH measurement may be used but allowance must be made for variations from the glass electrode values.

(d) Bufier agents-Maintenance of the pH values at the optimum or within the permissible range for a given alloy is complicated by the fact that the pH rises as the Mg dissolves during the treatment. In practical operation it is highly desirable for reasons of economy, to be able to treat a number of workpieces in the same bat-ch of solution. This necessitates frequent controlled additions of acid but the frequency of such additions is greatly reduced by incorporating in the solution a weak acid and one of its salts capable of buffering the solution within the desired pH range. Acetic acid is about the best and cheapest organic acid for the buffer mixture together with one of its salts of suitable cation as discussed below, particularly its lithium, sodium, magnesium, potassium or ammonium salt. Other weak acids of suitable dissociation constantand salts thereof with suitable cation, both acid and salt being of sufficient solubility, maybe used 'in place of or inaddition to the acetic acid and acetate. The buffer acid must not be oxidised by the dichromate solution. valeric, malonic, suocinio, glutaric, adipic, pimelic andphthalic acids are suitable. The buffer capacity or resistance of the solution to change of pH is roughly proportional to the concentration of the weak acid and its salt. Occasional major adjustments in pH may be appropriately effected by additions of sulphuric, chromic or nitric acids, ammonia, caustic soda or other base of. suitable cation. whichI prefer for .cold solutions are single or mixedacids and single or mixed salts of the acids and preferably of the same acids within the following group, namely, acetic, propionic, butyric, valeric, malonic, succinic, glutaric, adipic, pimelic, phthalic, the cations of the salt being those of lithium, sodium, magnesium, potassium, or ammonium. The acids and their salts whichl prefer for hot solutions are single or mixed acids .and single or mixed. salts of these acids and preferably, of the same acid, the acids being chosen so that the buffer agent is sufiiciently soluble at the temperature'of use, from the following group, namely, caproic, oenanthic, ma-' lonic, succinic, glutaric, adipic, pimelic, suberic,

Generally Propionic, butyric,

The acids and their salts,

benzoic and phthalic, and the'cations of the salts being those of lithium, sodium, magnesium, potassium, or ammonium. In cases where it is desired to secure the maximum possible protection, the sulphate ion concentration should not exceed the optimum region indicated under (1)) above, and in such cases the major pH adjustment should be made by chromic or nitric acid additions.

All salts used for introducing the dichromate and sulphate anions, the salt of the buffer acid and the base for pH adjustments must be chosen so that their cations meet the following requirements:

Firstly, within the pH range 4-6 they shall remain in stable solution in the presence of dichromate ions, sulphate ions and the buffer acid. Cations which 'do not satisfy the requirement result in the loss of dichromate, the formation of precipitated or colloidal matter, or change in pH with time, which render the process impossible or increasingly difficult in operation. To test the suitability of any given species of cation, a sulphate or dichromate containing this species is used in compounding the above solution. The resulting solution must be easily filterable through a filter paper without appreciable residue and on standing must not show a change in pI-I when measured in known manner.

Secondly, when magnesium alloy workpieces are under treatment, the cations shall remain in solution and shall not form metallic or undesirable deposits on the surface of the workpiece which reduce the protective value of the film as determined by a simple corrosion test referred to below.

The following cations, contained in readily available commercial salts or compounds, satisfy the stipulated requirements and are eminently suitable for use in my solutions: H, Li, Na, Mg, K or NR4. Examples of other cations which satisfy requirements are tetra methyl ammonium ions, tetra ethyl ammonium ions, and N methylpyridinium ions.

To sum up the afooresaid essential features and requirements, the invention consists in a process for the electrolytic protective surface treatment of magnesium base alloys in solutions characterized by the following features in combination, namely, a dichromate concentration not less than 0.05 mole per litre, a sulphate concentration not less than 0.05 mole per litre, and a hydrogen ion concentration controlled by a buffer agent or addition within the pH range 4-6, as measured by a glass electrode; the constituent salts being so selected that within this pH range theircations remain in stable solution.

The invention also consists in a process in ac cordance with the preceding paragraph, with one or more additional features, as stated more specifically in the claims below.

Referring to the accompanying drawings- Figure 1 shows the relationship between hydrogen ion concentration and percentage aluminium to illustrate the adjustment of the former to suit the aluminium content of different alloys.

Figure 2 shows the relationship between anode current density and corrosion rate.

Figure 3 shows the effect of time of treatment with reference to anode current density in respect of a number of metals.

Figure 4 shows the relationship of time of treatment against corrosion rate from the aspect of illustrating optimum conditions.

'In carrying the invention into 'e'fi ect, the alloy workpiece is immersed in the solution. and to :avoid confusion will hereinafter be called -the anode. A piece of another metalselected from those listed on the hydrogen side .of Mg in the standard electrode potential series, as given in Standard Textbooks on Electrochemistry, is also immersediin the "solution, and will hereinafter be called the cathode. This arrangement constitutes a typical electrolytic or galvanic cell such that when the anode and cathode are connected by a wire, current will flow through :the wire in the direction from the cathode to the :anode. Alternatively, the .anode 'andcathode may be connected to an external :source of current insucha manner that the current flows inthesame direction'as before. Underthe above conditions a protective film is formed on the surface of the anode. The electrolytic principles are the same whether the cell is short-circuited by a wire or supplied from an external source in the manner described and application of both methods for theprotection of metals has been known for a number of years. Ihave found that thedegree of protection when the method is applied to Mg base alloys using the dichromate solutions herein specified, depends on the magnitude of the current passing from anode to cathode, expressed as anode current density, in the manner shown by the curve in Figure 2. The curveshows that as the current density is increased beyond that corresponding to the knee of the curve, there is not .much improvement in protection. In general, this corresponds to about 10-20 amperes per sq, ft. of anode workpiece surface. .If the current density is increased much beyond this range there isa danger of the films becoming powdery in some cases. The lower current density range of the curve was obtained with the anode andcathode directly connected through an ammeter whilst the higher current density range was obtained with an external source of current. I have also found that when the cell is not supplied from an external source, the magnitude of the current depends on the nature of the cathode surface and changes with time in the manner shown in Figure 3.

Whatever the source of the current, I have found that as the anode current density increases, there is a tendency for the dichromate to be reduced in the neighbourhood of the cathode, the reduction compounds forming a sludge. This represents loss of dichromate and causes a drop in current efficiency. I have found, however, that this tendency depends on the material of the cathode surface and becomes less in the order, Al, Pb, Ni, and Cr and Fe, stainless steel, Ag, Au and Pd and Pt and :Rh, as is more clearly illustrated in Figure 3. .I therefore prefer the use of cathodes of stainless steel, silver or Cr or better still Au, Pd, Pt and Rh. Most of these metals may conveniently be in the form of quite thin deposits easily plated on to a cheaper metal. If desired the containing tank itself, constructed of the selected material, may be used as the cathode but in some cases where the workpieces are of highly irregular contour, it is preferable to use shaped cathodes to ensure greater uniformity of the protective film.

The nature of the connector between anode and cathode is of considerable practical im portance. If the cathode is composed of one of my preferred materials and the connector or its surface is composed of Ni or Cu, or preferably Sn, Pb, Cd, .Zn, Al or Mg, the connector'need not be insulated from the solution. For simplicity the suspension means for the anode workpiece maybe adapted to serve as the connector.

Al content. In some cases there is an optimum region from the point'of View of time of treatment as shown by the shape of the curve a in Figure 4. Curve a applies to an Mg alloy specimen containing 7.5% A1, 0.4% Zn, 0.2% Mn. Curve b applies to an Mg alloy specimen con taining 6% Al, 1% Zn, 0.2% Mn. Both specimens were electrolytically treated by connecting through an ammeter toa stainless steel cathode, the anodic current density being 5-8 amps. per sq. ft. The region of maximum curvature of curve a is about the point corresponding to, say, 6 minutes. That in curve b is about the point corresponding to, say, 11 minutes, although-with the alloy of curve ban increased time of treatment up to 30 minutes is not detrimental. In this latter case treatment prolonged beyond this time leads to a loss in adhesion of the film. Curve ashows that with the alloy of that curve an increased time of treatmentoth er things being equalreduces the protection, that is to say, increases the corrosion rate in respect of the effect obtained by treatment at a lower time corresponding to the knee of the curve. i

In any given case, the optimum value for any of the variables may be determined by means of accelerated corrosion tests representative of the particular service conditions the article is required to withstand. A salineimmersion or spray test of say 3% NaCl solution applied in well-known manner is generally regarded as representative of marine atmospheric exposure and it is on this basis that my ranges and the results in the figures have been established.

Variations in the ionic concentrations do not selected having regard to the degree of protection required.

For economy, drag-out loss is minimised by using the lower concentrations in any given range and by suspending the workpiece above the bath for a few seconds after treatment. The dichromate concentration tends to fall with use and further additions must be made as indicated by simple analysis. The sulphate concentration o-n the other hand rises when sulphuric acid instead Of chromic or nitric acid is used for occasional major adjustments'in pH.

The process may be carried out at room temperatures or at elevated temperatures. One advantage of a cold treatment is that drag-out losses are lower because in a hot treatment the workpiece itself becomes heated so that on Withdrawal from the bath the adherent film of'the solution rapidly evaporates thereby minimising drainage back into the bath.

For best results it is desirable to submit the workpieces to a cleaning pre-treatment. Solvent degreasing should be adopted if necessary followed by any of the standard published methods for Mg with the exception of sand or shot blasting or the use of hydrofluoric acid dipswhich reduce the protective value of the treatment. A quick pickle in cold dilute acid (nitric, sulphuric or acetic) or a mixture of these acids is agood method, but Where dimensional loss is to be minimised, chromic acid or" alkaline'cleaning should be used. In'the latter case it may be desirable in some cases to extend. slightly the protective treatment time. Alternatively, anodic polishing in accordance with the methods dis- 5 closed in British patent specifications Nos. 550,- 175 and 550,176 maybe employed as the pretreatment. v y p The following tables give a number of specific examples in accordance with which the invention may be carried into effect.

In all these cases the Mg base alloy workpieces are first given a pro-cleanin treatment as described above.

I Table I The solution in which the protective treatment is applied ismade up as follows:

Parts by weight Ammonium, sodium or potassium dichromate, singly or mixed 50-100 Ammonium sulphate, or if desired the equivalent amount of the other sulphates mentioned above 15-50 Acetic acid q. 10 Hydrated sodium acetate (or the equiva- I lent amount of ammonium acetate) 25 Water to make up to 1,000

For a Mg base alloy workpiece of composition according to specifications D. T. D. 118, A or 142 a higher degree of protection is secured by immersing the workpiece in the above solution of 5.3. touching the workpiece is a cathode plate-composed of stainless steel or mild steel or of copper,

nickel or mild steel sheet plated with a thin deposite of chromium or-rhodium.- The workpiece and cathode are connected to a supply of current in such manner that in terms of the convention previously mentioned, the current flows from the cathode to the workpiece. The supporting clamp for the workpiece is adapted as the electrical connector and that part of it in contact with the solution should be of mild steel, zinc, tin or cadmium coated mild steel or copper, but preferably aluminium base alloy. The portion outside the solution may be of the same material or any other usual electrical conductor material. Depending on the contour of the workpiece one or more fiat or shaped cathodesare used and suitably spaced so that the current can be adjusted to be reasonably uniform between 10 and 20 amperes per sq. ft. of anode surface. After treatmentin this manner for 3-5 minutes the workpiece is taken out, washed, dried, and

found to be coated with an adherentgreenish brown to blackprotective film which will also serve as an excellent basis for the usual-type of paintadapted for this purpose.

In cases where a'good but not such a high degree of protection is, sufiicient, the treatment time may be reduced to 1-3 minutes. Alternatively, the above concentration may be reduced by as much as 50% and the treatment tim kept at 3-5 minutes.

If any sludge is formed, which is least likely to happen with a precious metal coated cathode, and tends to adhere to the cathode surface, it can easily be removed by light swabbing. v Ifthe contour of the workpiece permits the cathode to be placed sufficiently close to give the same current density, which is assisted by heating the solution, an equal orsuitable degree of proaeeaecc current merely by connecting the workpiece directly to the cathode.

For a Mg base alloy workpiece of composition according to specifications D. T. D. 59A, 88B, 136A and 120A, a high degree of. protection is secured in identical manner except that the treatment time is increased to 5-10 minutes and the pH of the solution adjusted to suit the Al content of the alloy as indicated in Figure 1. In this figure the central curve represents the pH values for maximum protection. A tolerance is provided at each side of this curve as shown by the curves d and e. The shaded area between the curves d and e gives the working range for good protection. Thus for an alloy containing 6% A1, 1% Zn and 0.2% Mn the pH range should be 4.5 to 5.3, but preferably 4.9. For an alloy containing 7.5% A1, 0.4% Zn, and 0.2% Mn the pH should be adjusted to between 4.4 and 5.2 but preferably to 4.8. After treatment the workpiece is Washed and dried and found to be coated with an adherent black protective film which also provides an excellent basis for the usual type of paint coatings adapted for the purpose.

In cases where a good, but not such a high degree of protection is sufficient, the treatment time may be reduced to 3-5 minutes. Alternatively, the above concentrations may be reduced by as much as 50% and the treatment time kept at 5-10 minutes.

In all cases the same solution may be repeatedly used, occasional major adjustments in pH to maintain the desired values being made by additions of sulphuric acid. until the sulphate concentration reaches 200. parts by weight, when nitric acid should be used instead.

Table II This table may be taken to comprise the rec ommended range of ingredients.

Dichromate (CrzO'z). over 0.05 molar (-up. to

V saturation point) Sulphate (S04) over 0.05molar. Buffer mz'mture, e. g. acetic acid and sodium acetate, total acetate concentration as acetic acid plus sodium acetate together, over 0.1 molar CH3COO-. When other weak acids and their salts are used as bufier mixtures, these concentrations must be altered having regard to the Table I I I This table may be taken to comprise the range where the degree of protection is that corresponding to temporary protection of parts during fabrication, storage, transit or the like.

Dz'chromate (CI2O'7) over 0.1 molar (up to saturation point).

Sulphate (S04) 0.05-2 molar. (Note: For cases Where the dichromate is above about 0.3 molar, the sulphate concentration should be correspondingly increased.)

Bufier mixture, e. g. acetic acid and sodium acetate, total acetate concentration as acetic acid plus sodium acetate together, over 0.1 molar CHzCOO-. When other weak acids and their salts are used as bufier mixtures, these concentrations must be altered having regard to the equivalent weight of the acid relative to that of acetic acid.

pH 4-6 depending on alloy composition and time of treatment.

Time of treatment as in TablezII.

Table IV This table gives the optimum. composition for protective value of the highest order.

Dichromate (Cr207) 0.2-0.5 molar.

Sulphate (S04) 0.1-1.5 molar. (Note: For cases where: the. dichromate is above about 0.3 molar, the sulphate concentration should be correspondingly increased.)

Buffer mixture, e. g. acetic acid and sodium acetate, total acetate concentraition as acetic acid plus sodium acetate together, 0.2 to 1.0 molar. When other weak acids and their salts are used as buffer mixtures, these concentrations must be altered having regard to' the equivalent weight of the acid relative to that of acetic acid.

pH 4.6-5.9 depending on the and time of treatment.

Time of treatment.l30 minutes. depending on the alloy composition. Y

Table V This comprises an earlier type of solution which may be employed but which I do not now consider as good as solutions of Table IV.

A solution is prepared from the following ingredients in the proportions stated which are by weight:

alloy composition Parts Sodium dichromate Na2CrzO7.2H2O 100 Ammonium dichromate (NI-I4)2.Cr2O'7 100 Ammonium sulphate (NI-I4) 2504 100 Sodium acetate CHs.CO2Na.3I-I2O' Acetic acid CH3.CO2l-l 20 Water to 1,000

This solution is used electrolytically at room temperature and under such conditions forms a protective coating on magnesium and magnesium base alloy surfaces in a time not exceeding 10 minutes. For example, on a magnesium base alloy conforming to specification D. T. D. 118, a protective coating is formed in from one to two minutes. The composition may need to be adjusted slightly to suit particular alloy compositions. As the solution is not strongly acid, no special care need be taken with regard to containers and manipulation.

General By working substantially cold no special skill in manipulation is required to avoid undue loss of ammonia by evaporation with consequent change of pH. A high concentration of ammonium ions appears to be desirable, and apparently anions other than sulphate do not yield the desired results.

In connection with the bufiering of the solution, it appears that the buffer capacity or resistance of the solution to change the pH is roughly proportional to the concentration of the weak acid and the salt.

The new treatment provides an excellent basis for the usual type of paint coating applied to magnesium alloys. Adhesion of the paint film and corrosion resistance in the painted state are ood.

Electrolytic treatments do not necessarily give the same degree of protection with all alloys.

ll. For instance, whereas-the electrolytic treatment causes marked improvement in the case of magnesium base alloys containing aluminium, the improvement in the case of magnesium base alloys containing manganese additions only is but ll slight. These effects are illustrated by the following table based on a process in which acid cleaned specimens were connected externally with a stainless steel electrode immersed in the bath, and after treatment compared by standard 3% NaCl solution immersion test with specimens similarly treated but without the stainless steel. The following corrosion test results are the average of three specimens in each case.

, Time of Corro- Equiva- Alloying Additions Treatment Treatsion Test lent wt. ment Time loss Ordinary 5 mins 17 hrs 0. 43 9j {Electrolytidp 5 mins- 17 hrs. 0. 14 i Ordimry mins 2 gays." 0. 867 o mins 2 ays 0. 75 3; Mn to DTD Electrolytic." 2 mins. 2 days... 0.053 do 5mins 2days 0.060

During the electrolytic treatments the current density dropped from 19 ma./cm. just after immersion to 13 ma./cm. at the end of the treatment. On the aluminium containing alloys the dimensional change after five minutes electrolytic treatment was about 0.0002-3.

From these figures it follows that the improvement in resistance in the case of the aluminium vessels may be of steel or stoneware or any type of vessel suitable for any other chromate solutions. The solution has a long life, but can be replenished when the pH becomes too high by the addition of chromic or sulphuric acid.

I claim:

A process for the electrolytic protective surface treatment of magnesium base alloy which consists of treating said alloy in an electrolytic cell wherein said alloy is the anode,'the electrolyte in saidcell consisting of a solution in water of from 0.2 to 0.5 mol per liter of dichromate ion, from 0.1 to 1.5 mols per liter of sulfate ion, abufier mixture consisting of acetic acid and sodium acetate and beingadapted to maintain. the pH- of the electrolyte within the range pI-l4 to p116, the acetate ion being present in a concentration of 0.2 to 1.0 mol per liter, and the electrolyte constituents being so selected that within this pH range their cations remain in stable solution, the

current density in said cell being from 10 to 20 amperes per square foot of anode surface;

' GEOFFREY EDWARD COATES.

REFERENCES CKTED The following references are of record the file of this patent:

UNITED STATES PATENTS OTHER REFERENCES Alien Property Custodian Publication, 228,143,. May 18, 1943.

Clark, Determination of Hydrogen Ions, 2nd

ed. (1927) published by Williams 8; Wilkins C'O.,

Baltimore, Md., page 99.

Great Britain July l4, 193 

